1. Field of the Invention
The present invention generally relates to the art of audio/video data compression and transmission, and more specifically to a hysteretic synchronization system for a Motion Picture Experts Group (MPEG) audio frame decoder.
2. Description of the Related Art
Constant efforts are being made to make more effective use of the limited number of transmission channels currently available for delivering video and audio information and programming to an end user such as a home viewer of cable television. Various methodologies have thus been developed to achieve the effect of an increase in the number of transmission channels that can be broadcast within the frequency bandwidth that is currently allocated to a single video transmission channel. An increase in the number of available transmission channels provides cost reduction and increased broadcast capacity.
The number of separate channels that can be broadcast within the currently available transmission bandwidth can be increased by employing a process for compressing and decompressing video signals. Video and audio program signals are converted to a digital format, compressed, encoded and multiplexed in accordance with an established compression algorithm or methodology.
The compressed digital system signal, or bitstream, which includes a video portion, an audio portion, and other informational portions, is then transmitted to a receiver. Transmission may be over existing television channels, cable television channels, satellite communication channels, and the like.
A decoder is provided at the receiver to de-multiplex, decompress and decode the received system signal in accordance with the compression algorithm. The decoded video and audio information is then output to a display device such as a television monitor for presentation to the user.
Video and audio compression and encoding is performed by suitable encoders which implement a selected data compression algorithm that conforms to a recognized standard or specification agreed to among the senders and receivers of digital video signals. Highly efficient compression standards have been developed by the Moving Pictures Experts Group (MPEG), including MPEG 1 and MPEG 2. The MPEG standards enable several VCR-like viewing options such as Normal Forward, Play, Slow Forward, Fast Forward, Fast Reverse, and Freeze.
MPEG audio compression produces compact disk (CD) quality audio at very high compression rates. To save bits in the algorithm, a very simple 12-bit synchronization code, binary 1111 1111 1111 ("FFF" in hexadecimal notation) is used to designate the beginning of a frame of compressed audio data. However, unlike video synchronization codes, which will never appear anywhere else in a video bitstream, audio sync codes may be emulated in the data. These are known as false or invalid synchronization codes, and may occur anywhere in the audio data stream.
It is necessary to differentiate these invalid synchronization codes from true synchronization codes, which only appear at the beginning of an audio frame. If decoding of audio data is begun before proper synchronization is attained, the result will be a severely distorted audio presentation.
Once proper synchronization is attained, bitstream errors can temporarily cause misplaced synchronization codes, and result in loss of synchronization. For this reason, it is necessary to maintain synchronization in spite of occasional data errors. This is especially critical in audio/video applications in which the audio presentation must be synchronized in time to the video presentation.
The prior art method uses only the 12 bit synchronization code, which means that there is a significant possibility of initially synchronizing to a false synchronization code and thereby attempting to decode invalid audio data. Eventually, a valid synchronization code will be found, but until that time, the results are audibly unpleasant.
A 20-bit "header" immediately follows each valid synchronization code and contains basic information crucial to proper decoding. In some cases, some of these parameters are known in advance, and the extra bit values can be used to augment the regular synchronization code search. This is an improvement over relying on the synchronization code alone, but does not provide a complete solution.
The prior art method is susceptible to false synchronization codes, thereby producing bad output data. Also, attempts to re-establish synchronization through unrestrained synchronization code searches may cause a significant shift in the audio time base so that, for example, audio presentation is no longer synchronized with video presentation.
The synchronization code uniqueness can be increased by adding user bits. However, this is detrimental to efficient data compression, and prevents the scheme from being used in a universal or general purpose synchronizer implementation.
For these reasons, there exists a need in the art for a method of synchronizing an MPEG audio decoder which prevents false initial synchronization, while also presenting loss of synchronization caused by occasional data errors.